There have been different bearings provided to components having movement, such as a rotational or reciprocating movement (Rotor shaft or a moving shaft), for resolution of different problems liable to take place in movement of the component due to friction, such as problems of abrasion, damage, a problem of noise, and a problem of energy waste. In general, in the bearings used widely, there are sliding bearings, rolling bearings, wherein the sliding bearing is mounted to surround a shaft with lubricating oil provided to a portion in contact with the bearing, and the rolling bearing has rotatable components, such as balls or rollers, placed in a portion in contact with the bearing for minimizing the friction.
Those bearings used widely classically have a contact with the shaft taken place at any of the portion of the bearing without fail. Recently, use of a magnetic bearing is expanding in different fields for having no contact with the shaft, thereby minimizing the friction, really. As disclosed in the Korea Laid Open Patent No. 2009-0070178 (“A system for measuring cylindrical radial direction displacement of a magnetic bearing which uses capacitance and a method for determining whether the magnetic bearing has a fault taken place or not therein”, Laid Open on Jul. 1, 2009), the magnetic bearing is made to serve as a bearing by arranging a magnet having a strong magnetism or an electro-magnet around a shaft to make the shaft to be buoyant by magnetic levitation. Since the magnetic bearing has no contact with the shaft at all to make the friction to be zero, causing no abrasion or damage to the component, the magnetic bearing has many advantages, such as a long life time and a very low noise. However, in general, rather than designing the magnetic bearing to support the shaft only with the magnetic bearing, the auxiliary bearing which has direct contact with the shaft is provided in addition to the magnetic bearing for making securer supporting of the shaft, actually.
FIG. 1 illustrates a cross-sectional view of a rotor having a related art magnetic bearing provided thereto. As shown, in general, the rotor 1 rotated by a motor 2 has a plurality of bearings 3, 4, 6, 7, 9 and gap sensors 5, 8, 10 provided thereto. Since the rotor 1 has a columnar shape extended in one direction, in general, the rotor 1 has at least two radial bearings provided to an upper side and a lower side thereof respectively, and a thrust bearing provided to one side. In FIG. 1, since the radial bearings and the thrust bearing are the magnetic bearings 3, 6, and 9 for supporting the rotor 1 by the magnetic levitation, the radial bearings and the thrust bearing have no direct contact with the rotor 1.
When all of the rotor and the magnetic bearing system are in regular operation, the radial magnetic bearings 3 and 6 are in operation to support the rotor 1 by the magnetic levitation. However, if the system is stationary, since no power is supplied to the radial magnetic bearings 3 and 6 to generate no magnetic force, the radial magnetic bearings 3 and 6 cannot support the rotor 1. Consequently, for this case, the rotor 1 is provided with radial auxiliary bearings 4 and 7 in a mode of a general contact type bearing, such as the ball bearing. The radial auxiliary bearings 4 and 7 serve, not only to support the rotor 1 when the system is stationary, but also to support the rotor 1 even at the time of fault of the system for the rotor 1 to come to a safe stop of the rotation without damage thereto as far as possible. Accordingly, such radial auxiliary bearings 4 and 7 are essential components to be provided to the rotor. Though it has a mode different from FIG. 1, but has a structure similar to FIG. 1, a device is disclosed in Korea Laid Open Patent No. 2010-0054243 (Touch down ball bearing having a spring-damper system, Laid Open on May 25, 2010) of a mode in which the ball bearing is provided to a rotatable body in supplementation of the magnetic bearing together with the magnetic bearing.
In order to secure a space for providing the radial auxiliary bearings 4 and 7, the rotor 1 is designed to increase a length of the rotor 1 as much as a volume the radial auxiliary bearings 4 and 7 are to occupy. However, there is a problem in that, since the longer the length of the rotor 1, the larger the volume of the system itself. In addition to this, since there is a problem in that, since the longer the length of the rotor 1, the lower a critical speed of the rotor 1 in a bending mode, there is a limitation in increasing an operation speed of the rotor 1.